TWO-WIRE / FOUR-WIRE CONVERSION CIRCUIT

摘要

PURPOSE:To remarkably shorten the instantaneous disconnecting time of a signal which occurs when the action point of an amplifier exceeds the action level of the amplifier by means of a charging current by detecting the charging current of a capacitor connecting a feed circuit with the amplifier and absorbing it in a short time with the aid of a by-pass means. CONSTITUTION:Diodes D1 and D2 and transistors Q1 and Q2 transmit a signal to an output point 33 as a buffer amplifier. When the voltage V of the output point 33 rises by means of the outgoing of the PB signal of a subscriber terminal 1 and the charging current of a capacitor C begins to flow, the charging current is detected in the circuit including a current mirror circuit 34 and the transistor Q1 and a current is conducted to a power source-VC from another terminal of the current mirror circuit 34 through a resistance 35. The voltage of the resistance 35 by the current turns on a transistor Q3 and the charging current is bypassed to the power source-VC through the transistor Q3. As a time constant with the aid of the capacitor C and the resistance 36 is selected one figure smaller than the time constant with the aid of the capacitor C and the resistance 41 of an amplifier 4, the charging current of the capacitor C is reduced quickly and the instantaneous disconnecting time of the signal is shortened into t2.